What ailing pteropods tell us about climate change

By Larry Taylor

Jun 25, 2014 | 7:37 PM

The dissolving shells of the sea snails known as pteropods could be an ominous sign of another mass extinction. (Steve Ringman / MCT)

Pteropod, meaning "wing foot," refers to a group of animals that have neither wings nor feet as we usually think of them. Instead, these seagoing snails get their name from wing-like extensions they use to swim (the "foot" being the muscular portion of their body). They're unknown to most people, and recent news articles discussing ocean acidification and pteropod shells probably didn't grab the public's attention. But perhaps they should have.

These tiny snails make up the base of many oceanic food webs. Without them, everything in the food chain above them suffers, beginning with salmon and similar fish, then progressing to the species that eat the salmon and so on. Unfortunately, more than half of these snails collected in a recent survey showed extensive damage: Their shells are literally dissolving, killing them off in astounding numbers.

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The cause of this die-off, ultimately, is believed to be the rising levels of carbon dioxide. Leaving the chemistry details aside, about a quarter of the CO2 added to the atmosphere by the burning of fossil fuels is subsequently absorbed by the oceans, making the water more acidic. This change in ocean chemistry reduces the availability of a particular calcium compound that animals such as clams, oysters, mussels and our aforementioned sea snails need to build their shells. Without it, their shells are weakened, developing holes and slowly disintegrating.

This situation isn't entirely new for the planet; about 250 million years ago, the oceans endured similar changes in chemistry. Unfortunately, this past acidification event coincided with the Permian-Triassic extinction. Far worse than the dinosaur-killing extinction of the Cretaceous period, the Permian extinction wiped out more than 90% of marine species. The planet took millions of years to recover, the history of life was forever altered — and the whole thing may have been largely due to increased levels of CO2.

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That's the conclusion drawn by many geologists and paleontologists, including Jonathan Payne and his colleagues at Stanford University. Payne's research has helped to show that the chemical signatures of acidification are preserved in rocks deposited during the Permian, and that the species most sensitive to acidification were the ones most severely affected by this ancient biological crisis. The cause of the CO2 increase (volcanic eruptions in that case) was obviously different, but the results seem all too familiar.

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There have been a host of grim stories recently involving climate change. Reports from the Intergovernmental Panel on Climate Change and the National Climate Assessment confirm that climate change is being felt on every continent and in every ocean, and the effects are directly affecting our economy and health. Other reports demonstrate that global temperatures have been higher than the 20th century average for 350 straight months, that Antarctic ice loss has doubled in rate and that rising sea levels are unavoidable.

Meanwhile, confused pundits and media across the country pointed to the bitter Northeastern winter as evidence against climate change, when in fact research indicates that both the cold in the Northeast and the drought in California are ultimately linked to a change in the jet stream resulting from global warming. Finally, other researchers have already forecast a worldwide decline in shellfish harvests and fishery production due to climate change and ocean acidification.

The dissolving shells of pteropods are one more indication that the Earth is changing, and it happens to be changing in a way very similar to what caused the greatest extinction in history. What makes climate change different from so many other threats we've faced is the time scale over which it occurs; our fleeting life spans don't afford us a good perspective from which to assess the situation.

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Although the warming of the planet over the last few decades may seem like a slow progression, it's a mere instant in Earth's history, and the environmental changes we're causing far outstrip the ability of life to adapt. It's something akin to the biosphere being diagnosed with a cancer that turns terminal overnight. And like a cancer, the best we can do is prevent it; once the disease progresses, there's no sure way to cure it. So let's not overlook or dismiss these initial symptoms.

We're growing ever closer to pushing our home over the edge, perhaps into another mass extinction. If food webs collapse, all species will eventually feel the trickle-up effects, humans included.

Larry Taylor teaches biology at Arapahoe Community College in Littleton, Colo., and is a research fellow at Stanford University.

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